1. Field of the Invention
The present invention relates to a web tension control device provided at a vertical drier for cloth web, comprising in front of the lower cloth web inlet driven conveyor means, and the upper area of which comprises in particular driven guide rollers, and after the lower cloth web outlet of which are provided further driven cloth web conveyor means serving in particular as guide motor, wherein the drives of at least one of the conveyor means and, if necessary of the guide rollers are speed-controlled depending on the longitudinal web tension detected by sensing elements in the upward and downward guided cloth web.
2. Discussion of Prior Art
In vertical driers for cloth webs it is usual to guide the cloth web under tension between blast nozzles bilaterally loading them with drying air. The longitudinal tension in the upward and downward guided webs depends on one hand on the dead weight of the cloth web and on the other hand on the differential speed of the conveyor or guide means located at the beginning and at the end of each cloth web. In order to avoid overloading of the cloth web it is necessary to correlate the drive motors with a web tension control device.
The detection of the longitudinal tension in the cloth web running from the upper guide roller downward to the conveyor means located after the cloth web outlet of the drier is without problem. By means of a free-wheeling guide roller located in the web running direction in front of the conveyor means and supported at a pressure cell, it is possible to detect precisely the web tension without the portion resulting from the dead weight of the cloth web. Since in the downward guided cloth web the drying processes is almost stopped and since as a consequence the portion of the web tension resulting from the dead weight is practically unchanged, it may be considered as a constant value. Contrary to this, the detection of the web tension in the upward running cloth web is problematic. For different reasons a detection of the traction force by means of the bearing pressure of the first guide roller located in the cloth web running directions, would lead to errors: since in certain production adjustments the cloth web at the upper guide roller is not yet completely dried and therefore adheres to its envelope, the bearing pressure will be reduced due to the traction force produced by tne following driven guide roller. A driving motion of the guide rollers is always necessary when cooling rollers are used to which a cooling agent is fed through terminal heads. In fact, terminal heads impair easy rotatability of the rollers so that said rollers can no more be dragged along by the cloth web and have to be provided with a drive. Finally, the location of the bearing pressure measurement on the rollers inside the drier is problematic since the sensing elements are rapidly contaminated by the drier environment and become ineffective.
A tension detection directly in front of the lower drier inlet would therefore be ideal since at this point the described difficulties do not occur, and the portion of tension resulting in front of the upper guide roller from the cloth web weight varying with the drying degree of the web, is not integrated into the tension measurement. But there exist other problems for the tension detection in front of the lower cloth web inlet. In fact, it is barely possible to connect a measuring device for tension detection to a wet cloth web. For this reason, drive motors have never been adjusted to date depending on the tension in the upward running cloth web. Instead of this, the drive motors have been adjusted according to experimental values and by eye sight and with sufficient slack in order to avoid overloading. This resulted nevertheless in the drawback of a less precise guiding of the cloth web. Furthermore, it is known for cloth web winding processes to combine a speed control of the cloth web with a web tension control device (DE-AS No. 10 84 568). In this prior device the web tension is determined by the weight of a compensating roller and a variable load. The variable load is formed by a liquid contained in a reservoir conceived as a rocker member, the movement of which is coupled to that of the compensating roller. The position of the compensating roller determines on the one hand the winding speed and on the other hand the web tension. At constant rotational speed of the winding reel the winding speed is increased due to the growing winding diameter. By this action the compensating roller is lifted. This causes a displacement of the liquid contained in the reservoir and thus a relief of the web tension, but also a reduction of the cloth web speed. Thus is ensured that with increasing winding diameter the web tension as well as the winding tension is reduced. This means that in this prior device the web tension is not measured subsequently to varying the winding speed, but there occurs as a direct reaction to an increased cloth web speed an uncontrolled tension relief.